by Ammar Al Shamary and Gilgamesh Nabeel, Special for USA TODAY

by Ammar Al Shamary and Gilgamesh Nabeel, Special for USA TODAY

BAGHDAD - Iraq edged closer to the prospect of full-blown civil war Friday as a top Shiite Muslim cleric issued a call to arms against Muslim extremists who are continuing their military campaign to supplant the government.

In his sermon in Karbala, Sheikh Abdul Mahdi al-Karbalai urged citizens to resist the forces of the Islamic State of Iraq and the Levant (ISIL), an al-Qaeda splinter group that overran Mosul, the country's second-largest city, as well as other towns in the past week after encountering little resistance from Iraqi troops.

"Citizens who are able to bear arms and fight terrorists, defend their country and their people and their holy places, should volunteer and join the security forces to achieve this holy purpose," al-Karbalai said. "He who sacrifices for the cause of defending his country and his family and his honor will be a martyr."

In Baghdad, Iraqis were heeding the cleric's call. Former army officer Mohammed Saled, 45, was waiting to enter a military recruitment center that has been swamped for the past two days with men eager to defend the capital from ISIL militants, who have vowed to take the city.

"All Iraqis must get ready to fight those criminals who have no agenda except killing the innocent," Saled said.

As the representative of Grand Ayatollah Ali al-Sistani, leader of Iraq's Shiites, al-Karbalai threatened, with his sermon, to further divide Iraq along sectarian lines. Many Sunni Iraqis believe embattled Prime Minister Nouri al-Maliki has unduly favored his fellow Shiites, who make up the largest religious community in the country.

ISIL's goal is to establish a new Sunni country that would straddle the current border between Iraq and Syria, where ISIL forces are fighting President Bashar al-Assad in that country's civil war. The group was linked to al-Qaeda until internal disputes led to its separation earlier this year.

Experts believe the poor performance of the Iraqi army reflects how its Shiite officers are reluctant to fight against determined Sunni fighters in Mosul, Tikrit and other Sunni-dominated cities.

"ISIL has built up a head of momentum. Then, as it approaches other towns or cities, the security forces there panic, knowing that they are not well-equipped to fight or repulse the militants," said Matthew Henman, manager of IHS Jane's Terrorism and Insurgency Centre in London. "There is perhaps not a great sense of morale or a desire to actually engage and sacrifice lives in an attempt to hold territory in an area where they and the government are not particularly popular by any measure."

That pattern continued as ISIL kept up its blitz across northern Iraq. On Friday, the Islamic extremists captured Jalula and Sadiyah, two towns on the border with Iran, about 70 miles north of Baghdad. Upon entering the city, the fighters portrayed themselves as liberators protecting residents from the central government.

But reports of ISIL members terrorizing those who resist their rule have started to percolate out of the cities they control. On Friday, the U.N. High Commissioner for Human Rights Navi Pillay said the extremists might have killed hundreds and wounded as many as 1,000 people under their so-called protection.

"ISIL fighters, including prisoners they had released from jails in Mosul and provided with arms, have been actively seeking out - and in some cases killing - soldiers, police and others, including civilians, whom they perceive as being associated with the government," Pillay said in a statement.

In an effort to further demoralize government forces, ISIL has posted chilling online videos of its attacks on government sympathizers, including an incident where fighters decapitated an Iraqi police major in an unknown city to a soundtrack of Islamic hymns. The video could not be independently verified.

In response, the Iraqi government shut down Facebook, Twitter and YouTube in an attempt to stop ISIL from using social media to promote its cause.

The United Nations on Friday reported that 300,000 refugees had fled to Erbil and Duhok to escape the ISIL militants' violence - bringing the total number of Iraqi refugees in the conflict to 800,000.

A refugee in Erbil, Zainab Rasheed, described a chaotic scene at the city's bus station as people figured out whether they could make it to the safety of the capital.

"People were crowded to get tickets to go back to Baghdad," Rasheed said. "They are worried about their relatives there. They can't go by cars, as they are afraid to be killed by ISIL fighters."

Erbil and Duhok are in Kurdistan, a semi-autonomous region where troops are better organized than the central government's forces. About 500,000 Iraqis have already become refugees in other parts of the country as ISIL has acquired territory.

On Thursday, Kurdish troops occupied Kirkuk, an oil-rich city that Kurds have long claimed as their capital but Baghdad has controlled in recent years. Intending to prevent an ISIL takeover of Kirkuk, the Kurds moved into the city as Iraqi government forces abandoned their positions.

Adding another element of instability to the conflict as Sunnis, Shiites and Kurds staked out their territories, Iran's official IRNA news agency reported that ex-Revolutionary Guard members were ready to enter Iraq to vanquish ISIL. The agency said Iranian President Hassan Rouhani assured al-Maliki they could work together. Iran's theocratic government is Shiite.

"The Islamic Republic of Iran will apply all its efforts on the international and regional levels to confront terrorism," Rouhani told al-Maliki, according to IRNA.